In 1991, David Collison sent this figure to Canadian magic-square expert John Henricks, with no explanation, and then died.

It’s believed to be the first odd-ordered bimagic square ever discovered. Each row, column, and diagonal produces a sum of 369. The square remains magic if each number is squared, with a magic sum of 20,049.

No one knows how Collison created it.

UPDATE: Wait, Collison’s wasn’t the first — G. Pfeffermann published a 9th-order bimagic square as a puzzle in Les Tablettes du Chercheur in 1891. (Thanks, Baz.)

On June 28, 2009, Stephen Hawking hosted a party for time travelers, but he sent out the invitations only afterward.

No one turned up.

He offered this as experimental evidence that time travel is not possible.

“I sat there a long time,” he said, “but no one came.”

The tippe top is a round top that, when spun, tilts to one side and leaps up onto its stem. This is perplexing, as the toy appears to be gaining energy — its center of mass rises with the flip.

How is this possible? The geometrical center of the top is higher than its center of mass. As the toy begins to topple to one side, friction with the underlying surface produces a torque that kicks it up onto its stem. It does gain potential energy, but it loses kinetic energy — in fact, during the inversion it actually reverses its direction of rotation.

Entire treatises have been written on the underlying physics, and the toy has occupied at least two Nobel Prize winners — below, Wolfgang Pauli and Niels Bohr play with one at the inauguration of the Institute of Physics at Lund, Sweden, in July 1954.

University of Michigan mathematician Norman Anning offered this “non-commutative soliloquy of an introspective epistemologist” in Scripta Mathematica in 1948:

[(N + H)ow + (T + W)hat](I know).

Expand the expression and you get Now I know how I know that I know what I know.

Astronaut John Young smuggled a corned beef sandwich into space. As Gemini 3 was circling Earth in March 1965, Young pulled the sandwich out of his pocket and offered it to Gus Grissom:

Grissom: What is it?

Young: Corned beef sandwich.

Grissom: Where did that come from?

Young: I brought it with me. Let’s see how it tastes. Smells, doesn’t it?

Grissom: Yes, it’s breaking up. I’m going to stick it in my pocket.

Young: Is it? It was a thought, anyway.

“Wally Schirra had the sandwich made up at a restaurant at Cocoa Beach a couple of days before, and I hid it in a pocket of my space suit,” Young explained later. “Gus had been bored by the official menus we’d practiced with in training, and it seemed like a fun idea at the time.”

Grissom wrote, “After the flight our superiors at NASA let us know in no uncertain terms that non-man-rated corned beef sandwiches were out for future space missions. But John’s deadpan offer of this strictly non-regulation goodie remains one of the highlights of our flight for me.”